The present invention relates to a lithium secondary battery providing high output even under a low temperature condition, with an excellent cycle characteristic and reduced manufacturing cost.
Lithium secondary batteries are widely used as small and high energy density chargeable/dischargeable secondary batteries providing power supplies for portable communication devices and electronic devices such as notebook personal computers in recent years. Furthermore, with increasing concern about resource saving and energy saving against a background of international protection of the global environment, lithium secondary batteries are also expected as batteries for driving motors of electric vehicles (EV) and hybrid electric vehicles (HEV) for which the automobile industry is studying the development of a positive marketing campaign or as effective means for using electric power by saving nighttime electric power, and there is a strong urge to commercialize large-capacity lithium secondary batteries suitable for these applications.
Lithium secondary batteries generally use lithium transition metal complex oxides as positive active materials and carbonaceous materials such as hard carbon and graphite as negative active materials. Since the reaction potential of a lithium secondary battery is as high as approximately 4.1 V, no conventional water-based electrolyte can be used as its electrolyte, and therefore a non-aqueous electrolyte which is a lithium compound as an electrolyte dissolved into an organic solvent is used instead. A charging reaction takes place when Li+ in the positive active material is transferred through the non-aqueous electrolyte to and captured by the negative active material, and a reverse battery reaction takes place during a discharge.
Among such lithium secondary batteries, a relatively large capacity lithium secondary battery suitable for an EV or HEV, etc. preferably uses as its electrode body a wound type electrode body 1 as shown in FIG. 1 made up of electrode plates (positive electrode plate 2 and negative electrode plate 3) to which current collector tabs (positive current collector tab 5, negative current collector tab 6) functioning as lead wires are attached, wound around a core 7 with a separator 4 inserted in between to prevent the two electrode plates from contacting each other.
The positive electrode plate 2 and the negative electrode plate 3 are layers formed by applying respective electrode active materials (which refer to both positive active material and negative active material) to both surfaces of the current collector substrates such as metal foil. The positive current collector tab 5 and the negative current collector tab 6 can be attached to exposed metal foils at the ends of the positive electrode plate 2 and the negative electrode plate 3 at predetermined intervals using a suitable means such as ultrasonic welding during an operation wherein the positive electrode plate 2, and the negative electrode plate 3 are wound around the core 7, by sandwiching a separator 4.
Batteries used for an EV or HEV or the like may be required not only to have a large capacity but also to produce an instantaneous discharge of high current especially at the time of starting the engine or climbing a slope, etc. That is, the development of batteries having a characteristic of high in critical discharge current is required.
Here, focusing attention on the separator made of a porous film of polyolefin, etc. inserted between the positive and negative electrode plates, the separator does not always have excellent wettability for the non-aqueous electrolyte, that is, affinity or permeability, which is considered to cause not a little influence on characteristics of the battery.
JP-A-2001-6747 discloses a non-aqueous electrolyte secondary battery which uses paper having predetermined physical properties as a separator and improves battery characteristics. Even the non-aqueous electrolyte secondary battery described in the above publication does not provide sufficient performance as a battery for which a discharge of instantaneous high current is required. Furthermore, a battery which is supposed to be installed and used under a low-temperature condition, for example, a vehicle-mounted battery, etc. needs to demonstrate sufficient battery characteristics even under such a condition and therefore there is a demand for further improvements.